adjective

modest or shy because of a lack of self-confidence.

a diffident youth

shy,bashful,modest,self-effacing,unassuming,unpresuming,humble,meek,unconfident,unassertive,timid,timorous,shrinking,reserved,withdrawn,introverted,inhibited,insecure,self-doubting,doubtful,wary,unsure,apprehensive,uncertain,hesitant,nervous,reluctant,fearful,self-conscious,ill at ease,ashamed,abashed,embarrassed,shamefaced,sheepish,mim,mousy